Principal indications for dynamic, especially posteriorly performed, fixation are age- and/or disease-related decay (degeneration) of the integrity of the spine structures, inflammation and/or injuries in the region of the intervertebral disc, of the ligamentous apparatus, of the facet joints and/or of the subchondral bone.
Posterior dynamic fixation systems have the function of modifying the pattern of movement in the segment of spine in question that the pain caused by chemical irritation (material of the nucleus in contact with nerve structures) and/or by mechanical irritation (hypermobility) disappears and the metabolism of the structures is preserved or restored.
Clinical experience with existing posterior dynamic fixation systems as described, for example, in EP 0 669 109 B1 and in the manual “Fixateur externe” (authors: B. G. Weber and F. Mageri, Springer-Verlag 1985, page 290-336) shows that it is advantageous for a posterior dynamic fixation system to be flexible in respect of bending and rigid in respect of compression (buckling), shear and rotation. A system must accordingly be dimensioned for maximum deformation with regard to flexion and for maximum loading with regard to buckling, shear and rotation. In order to be able to combine these intrinsically contradictory conditions, it has already been proposed to manufacture the longitudinal members from a biocompatible high-performance plastics material. Because of the very low modulus of elasticity of the high-performance plastics materials compared to titanium and steel, the longitudinal members have to be of relatively thick construction compared to the longitudinal members conventionally manufactured from clinically used metal, which although having a positive effect on the resistance to shear and to buckling is detrimental to flexibility.
In addition, when using conventional biocompatible high-performance plastics material for longitudinal members, it is problematic that the plastics material in the mechanical fixing “creeps away” at the clamping sites after a relatively short time under the forces occurring therein, with the consequence that re-fixing or even re-implantation becomes necessary.
The possibility of being able to bend longitudinal members is of great importance especially in the case of posterior stabilization by means of pedicle screws, because the anatomical conditions are often such that the pedicle screws screwed through the pedicles and into the vertebrae are misaligned. In order that the longitudinal members can nevertheless be connected to the pedicle screws in as stress-free manner as possible, it must be possible for the shape of the longitudinal members to be matched in situ to the position and orientation of the pedicle screws. In the case of polyaxial pedicle screws, the bending adjustment can be limited to one plane, whereas in the case of monoaxial pedicle screws the longitudinal members have to undergo bending adjustment in three dimensions.
A further constructional form for a dynamic fixation system is proposed in EP 0 690 701 B 1. This last-mentioned system comprises a connecting rod, the extremities of which are fixable to two neighboring vertebrae and which has a curved central portion so that it is resiliency yielding within certain limits. Otherwise, the connecting rod is not modifiable in respect of how it is shaped.
Also, WO 01/45576 A1 proposes a dynamic stabilization system comprising a longitudinal member having two metallic end portions which are fixable in complementary accommodation apertures within the heads of two neighboring pedicle screws. Arranged between the two end portions is an articulation member which is resiliency yielding in the longitudinal direction, preferably made of resiliency yielding material. The two end portions of the longitudinal member are rigid. In addition to that articulation member, the arrangement of a resilient band between two pedicle screws is proposed, which extends parallel to the resilient articulation member. Otherwise, in the case of that arrangement too, the articulation member is, in respect of its longitudinal extension, predetermined during manufacture, that is to say it cannot be modified.
Mention should also be made finally of the arrangement according to FR 2 799 949, which is characterized in that the longitudinal member is in the form of a spring element, for example in the form of a leaf spring having a meandering curve shape.
Also, the longitudinal member in the case of the arrangement according to WO 98/22033 A1 comprises a spring element which retains its predetermined shape.
Also, EP 1 364 622 A1 describes a resilient stabilization system for spines which consists of a resiliently flexible connecting element or longitudinal member which can be passed through the apertures of a plurality of pedicle screws having offset aperture axes and anchored. That connecting element or longitudinal member should preferably be made of a resiliently flexible biocompatible material, preferably plastics material. Aromatic polycarbonate-polyurethane is mentioned as being especially advantageous. This is obtainable as a commercial product, for example under the Trade Mark BIONATE® from Polymer Technology Group, 2810 7th Street, Berkley, Calif. 94710 USA and CHRONOFLEX® C from CardioTech International Inc., 78E Olympia Ave., Woburn, Mass. 01801-2057, USA. The known connecting element or longitudinal member should have sufficient bending resilience about all axes of its cross-section to allow its insertion even into apertures of screw heads that do not lie on one axis but on a line of any desired course or that are naturally offset in various directions because of differing vertebral arrangements.
The integration of spring elements into a longitudinal member is also described in GB 2 382 304 A, U.S. Pat. No. 5,480,401, DE 42 39 716 C1, FR 2 827 498 A1, EP 0919 199 A2 or JP 2002/224131. It is common to all these last-mentioned arrangements, however, that they have a relatively complicated mode of construction, that being the case, more specifically, because the mentioned spring elements are integrated as additional components or structural units. In that prior art, the spring elements are not intrinsic to the longitudinal member.
The present invention is based on the problem of providing a device for dynamic stabilization of bones or bone fragments, especially vertebrae of the back, having at least one longitudinal member fixable to the vertebrae, which longitudinal member can be matched without complication to the very great variety of stations for implantation without the dynamic being lost and can be firmly fixed lastingly, especially to so-called pedicle screws.